The invention relates to a circuit arrangement for operating a discharge lamp with a high frequency current comprising
input terminals for connection to a source of low frequency supply voltage, PA1 rectifier means coupled to said input terminals for rectifying said low frequency supply voltage, PA1 a first circuit comprising a series arrangement of first unidirectional means, second unidirectional means and first capacitive means, said first circuit being coupled to a first output terminal N3 of said rectifier means and a second output terminal N5 of said rectifier means, PA1 inverter means coupled to said first capacitive means for generating the high frequency current, PA1 a load circuit comprising inductive means, second capacitive means and terminals for lamp connection, said load circuit being coupled to said inverter means and to a terminal N7 between the first unidirectional means and the second unidirectional means, PA1 a second circuit comprising an antiboost switching element S and shunting at least one of the first and second unidirectional means, a control electrode of said switching element being coupled to a control circuit for rendering the switching element conductive and non-conductive, PA1 preheating means for heating at least one of the electrodes of the discharge lamp comprising a first secondary winding, said first secondary winding during operation being part of a series arrangement shunting a first lamp electrode and being magnetically coupled to the inductive means comprised in the load circuit.
Such a circuit arrangement is known from WO 97/19578. The known circuit arrangement is very suitable to be powered from a regular mains supply generating e.g. a supply voltage having an r.m.s. voltage of 230 Volt and a frequency of 50 Hz. Because of the power feedback that is realized by the load circuit and the first and second diode, a relatively high power factor is realized with comparatively simple means. The circuit arrangement is so dimensioned that during stationary lamp operation there exists a balance between the amount of power fed back and the amount of power consumed by the lamp. Before the lamp is ignited, however, the lamp does not consume any power which can lead to the first capacitive means being charged to such a high voltage that part of the circuit arrangement, e.g. the inverter means, could be damaged. To prevent this, the circuit arrangement is equipped with the second circuit. In the known circuit arrangement the control circuit that is comprised in the second circuit monitors the voltage over the first capacitive means. If this voltage becomes higher than a first predetermined value, the control circuit renders the antiboost switching element S conductive, thereby disabling the power feedback. After the lamp has ignited, it starts to consume power so that the voltage over the first capacitive means drops below a second predetermined value, whereupon the control circuit renders the antiboost switching element S non-conductive thereby once more enabling the power feedback. The known circuit arrangement comprises first and second secondary windings that are part of the preheating means. These first and second secondary windings are magnetically coupled to the inductive means comprised in the load circuit. Both secondary windings are arranged in series with a capacitor and the resulting series arrangements shunt respective electrodes of the lamp. Before the ignition of the lamp the inverter operates at a frequency at which the impedances of the capacitors comprised in the series arrangements are relatively small. As a result a current with a relatively high amplitude flows through the lamp electrodes so that they are heated effectively. After ignition of the lamp the inverter operates at a much lower frequency so that the impedances of the capacitors are relatively high and the lamp electrodes carry a relatively small current. A disadvantage of the known circuit arrangement is that the current that flows through both lamp electrodes during stationary operation, though it is relatively small, continuously dissipates power in the electrodes thereby lowering the efficacy of the circuit arrangement.